Query on dollar

Dear All

want to know in details the following :

Our Company has a driver, he also looks after the foreign guests. Some of the foreign guest certain time gives him tip in dollar or euro. 

Now the query is, what is the best option to do with that.. he sometimes goes to some jewelery shop and get exchanged.. but he is not getting proper rate of that.. so pls let me know what to do in such a case..

Thanks & Regards,