On the importance of signing a written contract with the client...

Around two weeks I got contacted by a certain firm that needed a rather bulky translation performed with short turn-around time. So I agreed but (having had some bitter lessons throughout my translator`s career) and with some sixth sense whispering in me I knew I had to sign an agreement that would secure my rights and guarantee the payment. And yes, it came in handy, because after I sent the first half of the translation (never send the whole text because there is always a chance you will not see that money ever - and you cannot take every client to the small claims court, agree? It`s just not worth the time and effort), the client told me - "OK, now send us the other half- the remaining part and after we review it, we`ll send you the rest of the payment". But I said - "Nope, in the agreement it is clearly stated that the client sends the FULL amount and then the translator sends the rest of work". What could they do but agree to my terms?