[iggy1]

Can't you just check if cmdreason < 0? If so don't allow transaction.

pawn Код:

CMD:transfer(playerid,params[])
{
    new string[128],ID,cmdreason;
    if(sscanf(params, "ud", ID, cmdreason))
    {
        SendClientMessage(playerid,COLOR_ERROR,"USAGE: /transfer (id/name) (Amount)");
        return 1;
    }
    
    if( cmdreason < 0 )
    {
        SendClientMessage(playerid,COLOR_ERROR,"ERROR: Cannot tranfer negative amount.");
        return 1;
    }
    
    //... rest of stuff
    
    return 1;
}

EDIT: Changed your sscanf line to use the 'u' specifier too. Now you can do /transfer [name][amount] aswell as id.