The two stories (Sefer טל ירושלים)
The first מקור(source) of the Chai Rotel Segulah is from the Sefer שו"ת מהריל לא אבוש - טל ירושלים*
He writes, that he met two people (in Meron) serving a lot of Mashkeh and asked them
their reason for doing so.
One of them answered him that he was childless for 14 years. When he went to Meron
on Lag Baomer he made a Neder in a loud voice " He will donate 18 Rotel Mashkeh so
that Hashem should grant him a בן זכר that year." The people surrounding him all answered אמן.
The Ribono Shel Olam granted him his wish and he had a בן זכר that year.
3 years later he came back to Meron to cut the hair of the child and pay up his promise.
The second person had a similar story but being childless for 10 years.
I find it interesting that this Segulah works before you pay for it. You even have time 3 years later to pay.
In the Kunteres** of R.Osher Zelig Margolis zt"l who quotes the letter the Kedushas Tziyon Zt"l sent
to his Chasid to be Mazkir the childless couple " and when Hashem will help them with a child,
they will happily pay their donation". Here too, payment will be done only after he is helped.