I have seen this done before on larger deals ($5M+) and in my medical opinion, even if it's legal, it often isn't honest. :-)
The entire reason to provide a proof of funds to a seller is to credibly and authentically demonstrate your intrinsic ability to perform the financial obligations of the contract. By using an interim "guarantee" you are placating the seller in the hopes of securing financing in time to get final financing done, which may or may not happen. Surely this puts the seller in a bind should the end financing not come through. Usually people lawyer up at that point regarding damages, fraud etc. Please note, that I AM NOT saying that any of this is your intent, I'm only relating what I've personally seen occur with deals I've been involved in (note: I've never been the one to use a proof of funds letter in any deal).
In times past, when I've seen people pay to use a proof of funds letter, its use came with the stipulation that if the funds actually needed to be drawn down on, they would be used at hard rates over very limited terms.
Then there is the issue of how valid the letter for proof of funds is. I had one developer try to use a proof of funds letter from the Bank of Indonesia for somewhere in the neighborhood of $500M. That didn't end well.
Over all, I'd suggest you get an attorney's opinion. My medical opinion, being what it is, is not defensible in court. But my personal experience is such that if someone ever provides me with a proof of funds letter, I'm checking with the contact at the bank who gave the letter, and their supervisor as well to ensure that the buyer actually has the specific funds mentioned in the letter, under their complete and absolute control, and that those funds have a healthy degree of seasoning before I accept it as valid.
Best wishes,
Rob