Please Advise

I am torn. Do I stick to my guns and let the natural consequence be the teacher, or do I replace the lost money?
Just in case it matters, here is the scenario........
Harrison got money for his birthday. He has been carrying it around for days waiting, waiting, waiting until Tuesday (tomorrow) to spend it. He has had his eye in this particular truck for weeks now, waiting for his birthday money.
Today he was (again) carrying his money around. He wanted to go outside and I said "Hey, bud. You might want to put your money in your room so you don't lose it outside. You would be really sad if you lost it."
Well, he had it in his (unzipped) pocket and was running, chasing the neighbor girl, and felt it fall out of his pocket. I don't think she picked it up, but I can't be absolutely positive. I have caught her in a small act of dishonesty before. We did ask her and she said she didn't see it.
You know, as a Mom you want to fix everything. But which option will have serve him best?

KKS