“Buy this trouser and get two trousers free."
Here trouser has been used as singular & trousers for plural.
But check your dictionary and you will not find the word “trouser". You will get terms like trouser suit or trouser pocket. So trouser is used as an adjective and not as noun.
As a noun its always trousers.
Same rule is also applicable to words like scissors, forceps, pliers, shorts, jeans or pants.
Buy this scissors and get two forceps free.
I will buy a new trouser suit.