The phrase "to back the loan" means to provide something valuable or support as a guarantee to ensure that the loan will be repaid.
Sentences with «to back the loan»
Also, if there were further unforeseen circumstances where you couldn't pay back the loan in full when you said you would could cause friction and ill feeling. (123loanadvance.com)
Of course, to get those items back you'll need to pay back the loan with interest. (entrepreneur.com)
If you can't pay back your loan for any reason, contact your lender immediately to discuss alternatives. (simplethriftyliving.com)