From a "bribery" standpoint, I don't think your two examples are identical.
In the first you give the legislator $50,000. In exchange he agrees to vote how you want on a bill. That seems to be bribery and I would bet is against the law in most states.
In your second example, you agree to pay the legislator/lawyer a $250,000 retainer to represent you. Accordingly, there is at least an inference from your example that you are getting something in return -- $250,000 of legal representation.
If it could be proven that the lawyer did nothing for you for this "retainer", then I suspect a bribery charge could be made. However, if the lawyer did a substantial amount of legal work for you for which the $250,000 fee could be arguably justified, then I don't think there is any basis for a bribery charge.