A retainer question

I applied RIR LC (EB3) 03/05 and just received the LC in Nov. I have signed the following retainer with the lawyer:

"...This will acknowledge that I have retained XXX to assist me in obtaining immigrant petition services (I140). I have agreed to pay $2000 on or before the commencement of the work and $2500 upon the labor certificate had been granted by DOL....."

My question is that:
1 - If I do not want to apply for I140 using this lawyer any more, is that possible to get away without paying the $2500? It is written that "$2500 to be paid upon the LC had been granted ...". In other word, there is no limitation regarding when I must make the payment, right?