Can I provide proof of funds for another person and charge a fee?

[Charles Kamen]

This is purely hypothetical but I had an idea that I could explore if it's even legal.  My idea is to work with RE investors/buyers, and use my available funds to provide a proof-of-funds letter for their deals, and charge a fee of, say 1% of the value.  This would not be a loan, this would merely be a way for them to provide proof of funds to close the deal. 

Presumably this would be a scenario where they'd have other financing lined up already but not finalized, and they don't want to loose the deal waiting for the final lending approval  to come through.  It sort of fills the gap between closing the deal and getting the final financing approval.

Is this legal, does anyone do this?