it were West Ham United vs Real Madrid B in the 80/81 cupwinners cup
Its On Youtube (the return home leg) LOL .
they lost the away leg which is pretty funny, but they where also in the second division (same as rmB) at the time.
and this is similar to what i thought (but its not what i'm misremembering). but in this case, A reserve team and a first team both playing in the second division where eventually ,- The reserves got promoted to the top flight but the first team got relegated to the third tier.
sounds dodgy, funny, but dodgy.