Haven't scripted anything on LUA for years, but try this:
function onSay(cid, words, param)
pps = getPlayerPosition(cid)
if getPlayerItemCount(cid, 7910) >= 1 then
doPlayerRemoveItem(cid, 7910, 1)
doPlayerAddItem(cid, 7890, 1)
doPlayerSendTextMessage(cid, 25, "You...