Send SMS Credit to Your Friend's Mobile

I've been travelling a lot and noticed that in many countries (especially the developing ones) people use the pay-as-you-go mobiles. This means that when their credit runs out they are unable to make calls or send messages. However, what's nice is that they are still able to receive unlimited incoming calls and SMS messages.

In Mexico, for example, you can get a phone, a SIM card and 300 credits for about 30 bucks. This gives you unlimited incoming calls as well as 300 minutes of outgoing calls or SMS messages. You can purchase more credits as you wish, for example, 10 bucks will get you another 100. Most people, therefore, put more effort into composing a good message, in order to minimize the costs.

So wouldn't it be great if I could send a message to someone and include a credit with it? This would not only benefit the carriers (I'm getting charged twice, once for my original SMS to a friend, and second for my friend's SMS back to me) but also the users who are unable to communicate because of sudden exhaustion of credit.

This could be implemented in a number of ways. I think the easiest would be a simple keyword at the end of a message, for example: Hey Moses, I know you run out of credit so here is one, please let me know if you're coming to the party. Hope to see you tonight! <credit>

I haven't done much research on this but maybe the idea already exists.