You cannot set one type to another without a conversion or explicit cast. I assume that in the line:
currentmana = currentmana - manareq.Value;
that
manareq.Value
is the decimal type, so you need to cast it to integer thus:
currentmana = currentmana - (int)manareq.Value;
See
http://msdn.microsoft.com/en-us/library/yht2cx7b(VS.80).aspx[
^] for further information on explicit casting.