Anytime you accept assistance from a commercial tower you should determine weather it is covered service, in the case towing insurance i.e. Tow Boat/US, Vessel Assist, or Seatow (those are the ones I know of there maybe others), or is it a fee for service- salvage. If it turns out to be a fee for service you should have contract with the stating what his rates are, what he is responsible for and any other information that might be pertinent. He should give you an estimate of the cost but this will likely only a ball park estimate because as we all know thing on a boat rarely go as planned and this especially true when dealing with a salvage.
I know that the Boat/US web site had some excellent info the difference between towing and salvage and what to look for in both. If I can find the web address I will post in this forum.
Also be aware that if you do a good sam tow of another vessel it is illegal to except any compensation unless you are licensed by USCG and have a towing endorsement