Hello I need some assistance

Hello I need some assistance. I noticed two large transactions in my account that are marked as ‘pending’ from a merchant I don’t remember purchasing anything from. They haven’t posted yet, but they are reducing my available balance and making it hard to pay my bills this week. Does anyone know how long pending charges can stay in limbo? Also, is it possible to have them canceled before they post, or do I need to wait for them to fall off? This is stressing me out because it’s more money than I can afford to have tied up.