A legal contract for sale of one of my clients

Hello. I am selling one of my bookkeeping clients and have agreed a price with a buyer. The agreement is 50% being payable immediately upfront once my client has accepted the move, and the remaining 50% being payable over the following 9 months, with a clause that says the monthly payments will not be payable by the buyer should the client leave before the 9 months. Also with a proviso that the buyer will not sale on or give up the client (etc) without my paying the full balance or without my consent. It's not a major transaction so I don't want anything overkill, just a nice simple contract if possible please? Thanks! I need this very quickly - sorry!