return field from different table conditionally

I'm having trouble getting the values I want from my tables.
Below you find the DDL's for all tables concerned and some data to test with.

What I would like to get as a result is: for each billing_exact record the salesunit from account_ranges
if there is a match between billing_exact.lineamountmst and account_ranges.amountmst and otherwise
the salesunit from accounts_new.

Some joins that would certainly have to be made
billing_exact.fileid = exact_files.id
exact_files.dataareaid = accounts_new.dataareaid
billing_exact.reknr = accounts_new.accountnumber
accounts_new.accountnumber = account_ranges.accountnumber
accounts_new.dataareaid = account_ranges.dataareaid

And then there must be something like
if billing_exact.lineamountmst = account_ranges.amountmst then
return account_ranges.salesunit
else
return account_new.salesunit (consider this some kind of default value)
So the result would have to be 3 records (as there are 3 records in billing_exact)
and the salesunit from the appropriate table:

62, 700011, 53972, '2005-08-01', NULL, 139, '1y'
62, 700011, 53973, '2005-08-01', NULL, 159, '6m'
62, 700011, 53974, '2005-08-01', NULL, 278, 'pcs'

It can't be very hard to do this with a stored procedure but
can anyone help me out on how to do this with SQL alone?
Many thanks in advance.
Bart

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INSERT INTO "exact_files" ("id", "fname", "fdate", "fsize", "crc32", "statusid", "dataareaid")
VALUES (62, 'FR2005.DBF', '2008-02-01 09:06:00', 2326318, 'E2E1C53C', 3, 'lil');
INSERT INTO "billing_exact" ("fileid", "reknr", "transactionid", "transactiondate", "invoiceid", "lineamountmst")
VALUES (62, 700011, 53972, '2005-08-01', NULL, 139);

INSERT INTO "billing_exact" ("fileid", "reknr", "transactionid", "transactiondate", "invoiceid", "lineamountmst")
VALUES (62, 700011, 53973, '2005-08-01', NULL, 159);

INSERT INTO "billing_exact" ("fileid", "reknr", "transactionid", "transactiondate", "invoiceid", "lineamountmst")
VALUES (62, 700011, 53974, '2005-08-01', NULL, 278);

INSERT INTO "public"."salesunits" ("salesunit", "pg_interval")
VALUES ('pcs', '00:00:00');

INSERT INTO "public"."salesunits" ("salesunit", "pg_interval")
VALUES ('6m', '6 mons');

INSERT INTO "public"."salesunits" ("salesunit", "pg_interval")
VALUES ('1y', '1 year');
INSERT INTO "accounts_new" ("accountnumber", "dataareaid", "pool", "account", "dimension", "itemid", "salesunit", "poolid", "ignore")
VALUES (700011, 'lil', 'RN', 'A&C IMPOTS REABO', 'fracimal-2', 'fracimal', '1y', 'ALRN', False);

INSERT INTO "public"."account_ranges" ("dataareaid", "accountnumber", "amountmst", "startdate", "enddate", "salesunit")
VALUES ('lil', 700011, 159, '2005-01-01', NULL, '6m');

INSERT INTO "public"."account_ranges" ("dataareaid", "accountnumber", "amountmst", "startdate", "enddate", "salesunit")
VALUES ('lil', 700011, 278, '2005-08-01', '2005-08-31', 'pcs');

CREATE TABLE "public"."exact_files" (
"id" SERIAL,
"fname" TEXT NOT NULL,
"fdate" TIMESTAMP(0) WITHOUT TIME ZONE NOT NULL,
"fsize" INTEGER NOT NULL,
"crc32" VARCHAR(8) NOT NULL,
"statusid" INTEGER NOT NULL,
"dataareaid" VARCHAR(3) NOT NULL,
CONSTRAINT "exact_files_pkey" PRIMARY KEY("id"),
CONSTRAINT "exact_files_unq" UNIQUE("fname"),
CONSTRAINT "exact_files_file_status_fkey" FOREIGN KEY ("statusid")
REFERENCES "public"."file_status"("id")
ON DELETE NO ACTION
ON UPDATE CASCADE
DEFERRABLE
INITIALLY DEFERRED
) WITHOUT OIDS;
CREATE TABLE "public"."billing_exact" (
"fileid" INTEGER NOT NULL,
"reknr" INTEGER NOT NULL,
"transactionid" INTEGER NOT NULL,
"transactiondate" DATE NOT NULL,
"invoiceid" INTEGER,
"lineamountmst" NUMERIC(32,16) NOT NULL,
"creationdate" DATE DEFAULT ('now'::text)::date NOT NULL,
CONSTRAINT "billing_exact_pkey" PRIMARY KEY("fileid", "transactionid"),
CONSTRAINT "billing_exact_exact_files_fkey" FOREIGN KEY ("fileid")
REFERENCES "public"."exact_files"("id")
ON DELETE NO ACTION
ON UPDATE CASCADE
NOT DEFERRABLE
) WITHOUT OIDS;

CREATE INDEX "billing_exact_creationdate_idx" ON "public"."billing_exact"
USING btree ("creationdate")
WITH (fillfactor = 100);

CREATE UNIQUE INDEX "billing_exact_unq" ON "public"."billing_exact"
USING btree ("transactionid", (fnc_idx_dataareaid(fileid)));
CREATE TABLE "public"."salesunits" (
"salesunit" VARCHAR(10) NOT NULL,
"pg_interval" INTERVAL(65535) NOT NULL
CONSTRAINT "salesunits_pkey" PRIMARY KEY("salesunit")
) WITHOUT OIDS;

CREATE TABLE "public"."accounts_new" (
"accountnumber" INTEGER NOT NULL,
"dataareaid" VARCHAR(3) NOT NULL,
"pool" TEXT NOT NULL,
"account" TEXT NOT NULL,
"dimension" VARCHAR(16) NOT NULL,
"itemid" VARCHAR(20) NOT NULL,
"salesunit" VARCHAR(10) NOT NULL,
"poolid" VARCHAR(10) NOT NULL,
"ignore" BOOLEAN DEFAULT false NOT NULL,
CONSTRAINT "accounts_new_pkey" PRIMARY KEY("dataareaid", "accountnumber"),
CONSTRAINT "accounts_new_salesunits_fkey" FOREIGN KEY ("salesunit")
REFERENCES "public"."salesunits"("salesunit")
ON DELETE NO ACTION
ON UPDATE NO ACTION
NOT DEFERRABLE
) WITHOUT OIDS;
CREATE TABLE "public"."account_ranges" (
"dataareaid" VARCHAR(3) NOT NULL,
"accountnumber" INTEGER NOT NULL,
"amountmst" NUMERIC(32,16) NOT NULL,
"startdate" DATE NOT NULL,
"enddate" DATE,
"salesunit" VARCHAR(10) NOT NULL,
CONSTRAINT "account_ranges_pkey" PRIMARY KEY("dataareaid", "accountnumber", "amountmst", "startdate"),
CONSTRAINT "account_ranges_salesunits_fkey" FOREIGN KEY ("salesunit")
REFERENCES "public"."salesunits"("salesunit")
ON DELETE NO ACTION
ON UPDATE NO ACTION
NOT DEFERRABLE
) WITHOUT OIDS;

CREATE OR REPLACE FUNCTION "public"."fnc_idx_dataareaid" (p_fileid integer) RETURNS varchar AS
$body$
DECLARE
v_dataareaid exact_files.dataareaid%TYPE;
BEGIN
SELECT dataareaid INTO v_dataareaid FROM exact_files WHERE id = p_fileid;
RETURN v_dataareaid;
END;
$body$
LANGUAGE 'plpgsql' IMMUTABLE RETURNS NULL ON NULL INPUT SECURITY INVOKER;