CBC-MAC using predictable IV
You're saying we're looking at messages of length one block and a variant of CBC-MAC that uses a random IV. (I'm assuming we're looking at CBC-MAC for fixed length messages, i.e. we can ignore padding issues.) Then we have
$$am:=(ai,E(k,a\oplus ai))$$
To forge a MAC for message $b$ we can reuse the second part of the tag if we choose $bi$ such that $$b \oplus bi = a \oplus ai$$$$ \implies bi = a\oplus ai \oplus b$$

I.e., given $a$ and $am=(ai,t)$ simply output $bm=(a\oplus ai \oplus b,t)$.