You can have roundin problems, but this is a solution:

CREATE CURSOR test (field1 y,field2 I)
INSERT INTO test(field1,field2) VALUES (123456789012345.6789,123456789)

SELECT IIF(LOG(field1)+LOG(field2)>LOG(922337203685477.5807),$922337203685477.5807,field1*field2) as newfield;
from test;
INTO CURSOR new && CURRENCY VALUE OUT OF RANGE ERROR
BROWSE

A solution without rounding problem, but slow is

CREATE CURSOR test (field1 y,field2 I)
INSERT INTO test(field1,field2) VALUES (123456789012345.6789,123456789)

SELECT boundcurrency(field1,field2) as newfield;
from test;
INTO CURSOR new && CURRENCY VALUE OUT OF RANGE ERROR
BROWSE

function boundcurrency(yX,nY)
 TRY
   STORE m.yX*m.nY to yX
 catch
   store $922337203685477.5807 to yX
 endtry
 return m.yX
endfun

Fabio