CREATE TABLE #OVERPAY(OID INT, USERID CHAR(1), Rest INT)
CREATE TABLE #OPEN(IID INT, USERID CHAR(1), Amt INT, OpenROW INT)

INSERT INTO #OVERPAY(OID, USERID, Rest)
VALUES (1,'A',10),
(2,'A',15),
(3,'F',5),
(4,'H',20),
(5,'H',5)

INSERT INTO #OPEN(IID, USERID, Amt, OpenROW)
VALUES (1, 'A', 10, 1),
(2, 'A', 10, 2),
(3, 'A', 15, 3),
(4, 'F', 5, 1),
(5, 'H', 15, 1),
(6, 'H', 10, 2),
(7, 'P', 33, 1)

CURSOR

CREATE TABLE #map (OID INT, IID INT)
CREATE TABLE #usedIID(IID INT)
DECLARE @OID INT, @USERID CHAR(1), @Rest INT
DECLARE ov_cursor CURSOR FOR
SELECT OID, USERID, REST
FROM #OVERPAY

OPEN ov_cursor
FETCH NEXT FROM ov_cursor INTO @OID, @USERID, @REST

WHILE @@FETCH_STATUS = 0
BEGIN
    DECLARE @IID INT

    INSERT INTO #map (OID, IID)
    OUTPUT inserted.IID INTO #usedIID (IID)
    SELECT TOP 1 @OID, o.IID
    FROM #OPEN o
    LEFT JOIN #usedIID u ON u.IID = o.IID
    WHERE o.USERID = @USERID AND o.Amt <= @REST AND u.IID IS NULL

    FETCH NEXT FROM ov_cursor INTO @OID, @USERID, @REST
END

CLOSE ov_cursor
DEALLOCATE ov_cursor