Anyone else having issues with Santander bank?

So in 2015 our 2008 nissan rogue was declared a total loss in an accident, We did have gap insurance on the vehicle, and the gap insurance paid the gap between what the santander quoted the insurance company and what the insurance company decided to pay, however Santander sends us a letter in the mail saying the balance on the rogue was still $3846 and some change, We called them and explained to them that they misquoted our insurance company the payoff of the loan, and the balance is on them, So what they decided to do was stick that balance on our 2014 ford fusion we have an existing loan on, lie to us and say the error was on them, and we don't owe anything, blah blah blah, How we found out they did this, is when we got the ford the payments were $346 and went up to around $600 in the statement in the mail, the total pay off was higher than when we left the dealer, Is what they did legal?

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