I actually went bankrupt 8 years ago, and so I have now learned in a major way to not spend money that I don't have. I also heard a saying which I like (from Anthony Robbins I think) in regards to giving specifically and it goes - 'If you can't give a little when you have a little, then you won't be able to give a lot when you have a lot' ... He said what's easier ' Giving a $1 if you have $10 or giving $1000 out of $10,000 ? ... I realised from this little saying that I had actually had shame and fear in only being able to give a little, but the truth is, if that's all you've got, then what more can you give? Of course, the next step is to work on the emotions of why you don't have abundance as a lot of the guys have mentioned already. However, I know from my experience, I never used to 'give a little' because I felt ashamed it wasn't very much. And so, to confront my fear of giving, I started to give a 'little'! Giving a little actually brought up lots of fear about 'not receiving' and 'lack of money'.
After giving 'a little' for a while, I was able to enjoy it, and came to enjoy giving a 'little' without fear - and then I decided to see if was able to give a 'little more' than I was used to... again I felt the fear , and so on and so forth. Eventually, I then started to experiment giving when I felt inspired and desired to give, initially lots of fear would come, but I gave anyway and felt the fear, but then I also felt the joy. And so now, giving has become much more enjoyable, in fact it feels awesome to give.
I realised I often had a heart of gratitude, but my fear of lack, would prevent me from parting with money or things. Now, I am much more comfortable in giving, I'm not totally perfect yet, still some fears, but I'm better than I used to be.
The other thing I realised when I gave a little if only I had little, was that the receiver could feel that I was being sincere in my offering, and they would be grateful for the gesture, even if it was a small amount. I know that when someone 'gives' me money, I'm blown away, no matter the amount, as I know they don't have to, they could have chosen not to, so it feels like such a gift. Obviously there is danger of being taken for granted, but you can normally feel that, and then act accordingly.
So I'd say don't be afraid of giving a little if you only have a little. And also, if someone is offering something for free / donation ,then they generally mean it, so take up the offer. And yes, if in the future, you happen to have a windfall of cash, you may then wish to send some money to those you have appreciated in the past. I also agree with Nicky's response to the suggestion that was made to you in regards to 'helping out' at the seminar in other ways if you don't have money. As Nicky said, this is just then a barter, and also, you will probably be helping Jesus and Mary out of guilt rather than love.
Anyway that's my two cents worth

There is more I could say, but I think Jesus sums the topic up well in the links below

(Money Link)
https://www.youtube.com/watch?v=rx44wp4VOnM