The sums go like this: ( these are just simple hypothetical figures)
Value of property: 500,000.00
Amount required to redeem mortgage: 450,000.00
Difference 50,000.00
Take off solicitors and auctioneers fees 10,000.00
Nett profit on sale 40,000.00
If you want to buy at 750,000.00, you will have transaction costs ( solicitors) so say a total of 755,000.00, you will have a deposit of 40,000.00 ( and any other savings) so you will need to find 715,000.00. Would a bank lend you that amount of money? They very likely will only lend you 90% and they also need to decide if you can meet the repayments.
You probably need to talk to a lender to see how much they would lend you to really clarify your figures.
mf