The concept of a "mortgage" is usually just signed as "LOAN."
If you need to differentiate between a mortgage and some other loan you can do the sign HOUSE just before doing the sign LOAN.
The sign LOAN is a "directional" sign, by that I mean it is an "agreement verb" (and by that I mean) it can show who is the subject and who is the object. For more information on the "LOAN" sign see: LOAN
My point though here is that you might see people express the concept of "mortgage" using either version of the sign for LOAN, (the version that moves toward the signer or the version that moves away from the signer) depending on whether you are the person getting the loan or if you are simply discussing loans in general.
For example:I/ME WONDER I QUALIFY BORROW BUY HOUSE?!
(The sign BORROW starts away from the body and moves in toward the body.)
Which could be interpreted as: "I wonder if I'd qualify for a mortgage?"
Oh sure, you could add the IF/SUPPOSE sign, but realistically the sign WONDER means "wonder if" and/or "wonder about" and thus you don't need to add the additional sign "IF."
Other example:
IT #BANK WON'T LEND-you! YOU NONE JOB!
(The sign LEND starts near the body and moves away from the body. Or it starts near an established subject referent (lender) and moves toward the object (borrower)).
Which could be interpreted as "The bank isn't going to approve you for a mortgage because you don't have a job." Oh, sure, you could fancy it up with the actual sign for APPROVE and/or BECAUSE. You could even use a rhetorical "WHY" if you felt like it. But none of that is my point. My point is is that the LEND/LOAN sign moves from the body outward whereas the sign we label as "borrow" starts "away" from the body and moves inward.
In English you just say "mortgage."
In ASL the easy way to keep it straight is to remember that where the sign ends up indicates where the money ends up.
Also see:
15-Year Fixed Rate Mortgage
APR