You could get a letter of pre-approval or, if you want something even more firm (so you don't have to put 'subject to financing') you can ask them to take it a step further (can't remember what that letter would be called).  

Some blocks are less desirable and some are very desirable, one mile could be a huge difference.

It was a long shot but I wrote a long (well written) letter and they sent the check back.

A few blocks one way or another can make a difference.

If that's what she wants, issue a permission letter, stating she is responsible for all installation costs. 

Like send them a letter saying "Hey just wondering if you'd be able to do seller financing, here's what I can offer you and why it works out for you?

The attorney got a Letter of indemnification from a successor bank and we settled on time. 

Bought a 4/3 block house for $ 15,765,Went to the clerk's office and bought it,registered the deed, and called my Realtor.Drove toward the house, stopped at Home Depot and bought new locksets.Met Realtor at the house... drilled the locks for access, and as I was changing locks, she took pictures.

So I found this great HUD Owned large 3 story Victorian Col on corner double lot, one block from waterfront n view from 3rd floor with open and enclosed porches,  in the best part of town I grew up. 

Pre-approval letters are worthless sometimes.