I received an email from a prospective "client" in broken English. We have been going back and forth about services for a while and he wants me to text him. Is it possible for him to charge me some scam fee if I text him or otherwise do something unauthorized?
Yes, it is possible. There are pay-numbers for sms and calling and if you happen to call to that number or send sms, you are charged with extra than usual call/sms. Often these numbers are used for charity by phone or call-centers and users are informed about rates of these calls. However, they can be used for malicious intent.
To avoid it: 1) Don't call or send sms to numbers you don't know. Usually these numbers are different in format from standard numbers, usually start with 900. See here - http://en.wikipedia.org/wiki/Premium-rate_telephone_number 2) Some phone providers allow clients to block premium-rate phone calls, so that when you actually try to make call/sms, it fails.
On side note, also there are some services that allow users to subscribe, so that they receive something every week or even every day (funny pictures, ringtones..). There have been cases when that is not stated clearly and users are tricked in to subscribe and don't notice that they are paying regularly for services that they didn't intend to use.
Actually there was a scam like this going on here in Belgium. People would send you a text and you would end up replying to pay-number in a foreign country. This ended up in paying a rather high fee.
Now he wants to text, I do not see a difference in texting or email. If he wants to talk let him call you up. I would also advice you to contact your provider and explain them the situation. My provider has a setting where they can disable you from calling pay-numbers, maybe you should inform if yours has one too?