I really can’t see a problem. The copyright of the design is automatically yours to do what you like with it. If you wish to sell this design on to the mining company that is entirely up to you.
As far as the CAT company is concerned, buying your signs does not give them any copyright on the design you produced. (I’ve bought lot’s of Beatles records in the past – but that does not give me any copyright on their songs).
I presume the CAT company must have passed your name onto the mining company in order that they could contact you direct – so this would imply that they regard the design as being yours and not theirs.
I think the fact the mining company liked your design so much that they now wish to adopt it formaly as their logo speaks volumes for your ability as a designer.
I’m not suprised they liked your design – it’s excellent. 😀
To answer the first part of your question:- I think the system you have is quite reasonable and generous. I work in a similar way, and have provided good customers with copies of their artwork free of charge in order that they may use a design on a letterhead for example. This policy is possibly not very business like (perhaps we should be charging in instances like this) However, I think the goodwill (resulting in repeat business and referals etc.) it helps to create between you and the customer offsets the fact that you are effectively giving away your design.
If you were to change your policy I do think some customers on being charged for the design would not then think twice about taking it elsewhere to be used on signs. Whereas the customer who you have a good relationship with, who understands that you are being helpful – will automatically use you for any future signs. In other words your policy is probably quite reasonable. If it works – don’t fix it.
I’m sure there are many who will say the customer should always pay for the right to use a design on something else, and that by not doing this you are losing a valuable source of income. However, if you adopt this policy, you may also lose valuable goodwill which is very hard to put a price on.
Just my thoughts