Learn Arabic (Modern Standard): كنت سألغي المعاملة.

Placing the past tense of كان before a present tense verb normally creates the past continuous ('I was canceling'). Adding the future prefix سـ to that present tense verb shifts the meaning to an intended future action from a past perspective, resulting in 'I was going to cancel' or 'I was about to cancel'.

Why is the present tense verb pronounced with a damma on the first letter (أُلغي) instead of a fatha?

This is a Form IV verb, from the root ل غ ي. In Modern Standard Arabic, the present tense prefix of all Form IV verbs takes a damma. Since this is the first person singular ('I'), the prefix is an alif, making it أُلغي (ulghī). With the future prefix attached, it is pronounced سأُلغي (sa-ulghī).

Does the word المعاملة always mean 'transaction'?

Not always. It is the Form III verbal noun from the root ع م ل (to work/do) and frequently means 'dealing with' or 'treatment' (such as how you treat a person). However, in administrative, official, or financial contexts, it is the standard word for a 'transaction', 'procedure', or piece of 'paperwork'.

What case ending does المعاملة take in this sentence?

It takes the accusative case (mansub) with a fatha at the end, making it المعاملةَ (al-mu'āmala-ta). This is because it is the direct object receiving the action of the verb ألغي ('I cancel').