When there were property requirements to vote in the United States were people who were buying their houses with a mortgage eligible to vote?
Yes. So long as you could prove ownership of the property debts against the property would not limit you from participating in politics. For one example consider Thomas Jefferson
But it is important to remember that the current mortgage system has only been around since about 1949. Prior to this most Mortgages would have been for a much shorter term typically 3-5 years during which time only interest would be paid with the principle being due at the end of the term.