devrais vs dois in owing money

Question

In the next to last sentence would it be more polite to ask "Je vous devrais combien?" instead of "je vous dois...."

Accepted Answer

Bonjour William,
You have got the structure of the sentence correct, but propose an inappropriate (conditional) verb form.
The structure is to owe something to someone  --  je vous dois combien? Here we have the subject Je (first person conjugation of devoir) dois something (combien) to (preposition) vous (you) indirect object. 
The work has been carried out to your satisfaction and you simply want to know how much your mechanic is owed. Not could be owed.
Bonne journée
Jim