Tenant refuses to pay rental application processing fees

I have a situation where a prospective tenant applied for one of my rental properties using the TAR(Texas) 'Residential Lease Application' form. I processed the application without collecting any fees upfront, I paid for tenant screening service from my pocket. I thought I will collect it later along with the deposit. But the tenant had a change of mind and went with a different property. And now refuses to pay the $100 application fee( $50 per app, 2 applications). He says that I did not ask him for the fee earlier, so he concludes he does not have to pay .  On page 3 of the application form, under FEES, the box 1 is checked which states '$50 to be applied to the security deposit upon execution of a lease or returned to Applicant if a lease is not executed'. Since the applicant withdrew after I had already spent money on his tenant screening, isn't he supposed to pay ?

Any ideas how I could make them pay this fee of $100 ? Could I threaten him with hiring a collection agency who can report to credit bureau ?   Thanks much for your advice.