An interesting case.
Dealing with the process that they should follow first, you should have been suspended on full pay pending investigation by the employer, who should then invite you to an investigatory meeting to enable you to put your side of the story. What normally happens then is that that your suspension will continue whilst the employer follows up anything it feels it should be after the interview with you, then they will reach a decision, which will be communicated to you. You should be offered the right of appeal (if the decision is one of disciplinary action) and the appeal should be heard by someone independent of those reaching involved in the process that far. Basically the appeal person (who is normally a more senior manager) looks at the process followed, the evidence and the decision. You can't put more evidence in at that stage.
The decision is not merely one of dismiss or not - it could be anything in-between to include final written warning or written warning (or no further action).
What your task should be in this process is to ensure any decision is no graver than a FWW.
Employment Tribunals follow a dismissal (and you don't 'sue' at an ET) and what they look at is to check for a fair PROCESS and a fair DECISION. Far more ET decisions go in favour of the employee at ET because the employer failed to follow a fair process (even though the dismissal reached was ultimately probably correct).
The problem the employer has got is finding enough evidence 4 years later. Personally (and this is only my personal opinion) I don't think it is good enough to merely say the money-tracking system is bullet-proof so you are guilty QED. They have to talk to staff, and that certainly should mean the retired manager. The fact that you have something in writing from him, I would keep very quiet until the investigatory interview. At interview, then seek to find out what steps they have taken to check out the circumstances. This should have involved both speaking and the retired manager and other employees who have bought stuff.
I'll stop at this point, but do keep in touch with us if you want further info prior to any investigatory meeting (to which you should bring a friend who should write notes of the meeting on your behalf) - it is too early to be talking about ET and compensation yet - you haven't been dismissed yet.