Need a forumla to look up data that is close to another cell value with multiple entries

[pljnyc]

A B C D
1unit Number Billed System System Date
2615695 546456


cell a2 = unit number = 615695
cell b2 = billed = 546456
cell c2 = system = need formula
cell d2 = system date = need formula

I cannot figure out if a formula for this, I have the data above I need to look on another spreadsheet that has multiple entries for the same unit numbers, I need to find on the other spreadsheet this unit number with a reading closest to the billed field and the system date.

Is this possible