Uncertain graduate study plans due to timing problems with visa and professor's funding I applied for the university of Alberta.Fortunately I have got funding opportunity through my prospective professor's grant. I have applied for a visa expecting to go with the January intake, and waited for 6 months to get the entrance visa. As my visa came late I decided to start in the spring semester (May). I contacted my professor 3 weeks ago (in March) and he said he had a good job for me if I can come by May. 

Yesterday, I finally got my visa and thus I contacted him again. My prof told me not to come for the May semester as "I have to give your project to other one since your visa delayed". And it seems he is on a sabbatical leave.
He says he does not have funding for me until September and my funding status will be clarified in 3 months. I already have my visa and I have done all the things that I needed to go. Now I am up in the air.

If I accept that I come in Aug/Sep, how can I make sure that he is going to fund me in 3 months? What he decided devastated my plans for starting my term in May. How can I ask for a guarantee? I think he should assure that he will give me the fund in 3 months since I have been waiting for 1 year now.

+from India and I need an entrance visa.
Why he acts like this? 


We cannot know, but there are plausible reasons. Some of my funding is available in the form "use it [in this time period] or lose it." If I have a project like that and a student indicates that it is likely (s)he cannot come, I will give that project to someone else rather than lose the money entirely. Here the prof has to make an educated guess: (s)he wants to avoid the situation you are in, but we have all been let down by students who promised they would come, but at the last moment did not. So apparently your professor made the guess that you would not be able to make it. 


  how can my I make sure that the professor will give me fund after 2 months?


If you have a contract, but apparently you didn't have one, so that suggests that you are from a country where it is common to sign such a contract at the very last moment or where funding is not considered a form of employment and thus there is no employment contract. As you can see that is hugely dependent on the local laws and customs. In those cases, the answer is: you cannot be sure. Since we don't know which country you are in, there is little more we can say.