Rule reminder
Exceptive clauses (a meno che, tranne che, salvo che) describe a circumstance that would PREVENT or EXCLUDE something — they express an exception. Restrictive clauses (purché, a patto che, a condizione che) describe a condition that must be MET for something to happen — they express a requirement. The two types have opposite logical orientation: exceptive = 'X happens, UNLESS Y'; restrictive = 'X happens, ONLY IF Y'.
You want to say: 'I'll sign, but only if you lower the price.' Which sentence is correct?