两种译法都是对的，具体是委托费还是预付费要看具体情况。

retainer如果指律师费，可译为“委托费”“诉讼费”“律师费”。如：

This Court of Appeal case deals with the effect of a breach of the Solicitors‟ Practice Rules on the solicitor–client retainer, and the extent to which a paying party (the defendant) can rely on the breach to avoid paying the claimant‟s costs.

这宗上诉法院的案件，是关于违反《事务律师执业孚则》一事对事务律师与当事人之间的聘用事宜有何影响，以及付款一方（即被告人）在甚麽程度上可以凭借违反执业孚则一事而免付申索人的讼费。

Only persons lacking sufficient resources to pay court fees or lawyers’ retainers are eligible for legal aid.

只有缺乏足够的资源而无力支付诉讼费或律师的委托费用的人才有资格获得法律援助。

如果指其他费用，可译为“预付费”或“预付聘金”。如：

The Client is obliged to pay a Retainer which is set out in part C of the Confirmation of Instruction.

客户须支付指示确认书 C 部分规定的预付费用。