My advise to you, don't take her back to your place even for a visit. You go and visit her at the new owner's place instead. Don't make her more stress by moving her here and there. You have decided to give her up. The only thing you can do if you really miss her, visit her instead bring her back to your place.

In the first place you should not have given your cat up for adoption due to 'on heat' problems.
You should have just gotten her spayed and your problems will be solved.

I agree with what Listiani said.  You will stress her up by bringing her back.  Just visit her at the owner's place.  I am sure a short visit would be okay. 

What a good deed to feed the stray cats.  Reasonable food like Star Pro would be great to feed the strays.  This is what my neighbour is doing.  For 18 kg's (big bag) you can buy it at RM135.00.  If you calculate its even cheaper than Frieskies or Whiskas however the food content is much better than Frieskies or Whiskas.  Others next on the list would be Sportsman Pride, Vet Master etc.

Try holistic food like Evo, Ancana or even ANF.

Hmmmm..... if she dont want to let you inside the house, at least she can bring the cat outside the gate and let you see it for a while unless.... she has something to hide     So difficult also this owner.....    Dogs will bark at anything and not only you so ask her to be a little bit considerate la.....

You can try to check out any pet shops nearby to your house and see if they are selling repack Star Pro or other food.  Sorry to say that I have never heard about Natural Trainer 

maybe she dont want the cat any more so ask u to take for one day. then she hilang terus

that sounds suspicious
I think I agree with dfcloud. did she say exactly why you can't enter her house?

I also don't think it's a good idea to move the cat around. best is to visit her.

to choose good cat food, you have to read the ingredients, especially the 1st ingredient in the list which is the highest amount. 1st ingredient = meat, is better than corn/rice/cereal