If you were holding them for a friend, knowing they were stolen, you were a party to the crime, making you legally as guilty as the person who actually stole them. You are probably sunk on those charges.
I've copied this directly from the Theft Liability statute:
(1) under Section 134.003(a), from a person who commits theft, the amount of actual damages found by the trier of fact and, in addition to actual damages, damages awarded by the trier of fact in a sum not to exceed $1,000;
This makes you liable for the damages (of which there were none because they got the pants back). The judge can penalize you up to $1000.
As far as Spencer's not detaining you, that doesn't matter. You don't get a free pass simply by making it out of the store.
If you go to court the judge could impose higher penalties than what they are asking for.
You should definitely get copies of whatever you signed.
If you don't think you can successfully defend yourself against the charges (from what you've said, it doesn't sound like you can), then I recommend that you contact Macy's and Spencers and offer them something less than what they are asking. Tell them all you can afford is $100 and negate them down. $350 is their first offer to settle, and they are most likely willing to settle for less.
If that answers your question, please click "Accept." If you have any follow up questions, feel free to ask.