Finding a number in a column and returning the cell 5 columns to the left

For example, I want to find "138000" in column M of another spreadsheet, and return the value in column H on the spreadsheet. I thought "=offset(VLOOKUP(A1,'105'!M:M,1,false),0,-5)" would work, but it doesn't. Thanks.

No, Vlookup returns a Value, not a Range Object that can be offset from.

Try

=INDEX('105'!H:H,MATCH(A1,'105'!M:M,0))