What is the ruling on not going to pay for a First Aid diploma certificate that was received more than a year ago?

If the advertisement stated that attendees of the course would receive specific certificates, including a syndicate certificate, for a specified amount, and a reduction in the amount occurred, then you are entitled to receive all the advertised certificates unless they explicitly state that the reduction deprives the participant of a specific certificate. The original understanding is that whoever attends the course receives the mentioned certificates. A reduction does not diminish what a full-paying participant receives; otherwise, its advantage would be negated. If the reduction necessitates a decrease in what is offered to the participant, then this is a new contract that is not valid unless explicitly stated. Allah Almighty says: ﴿O you who have believed, fulfill [all] contracts.﴾ And the Prophet, peace be upon him, said: "Muslims are bound by their conditions." There is no blame upon you in obtaining the certificate, and no weight is given to the institution's assumption that you knew the reduction would deprive you of it, as the shortcoming is theirs. And from the rules of Fiqh: "No consideration is given to a clearly erroneous assumption."