Can a Jew make a deal with a restaurant or water bottle vendor in a park that when he comes by on Shabbos, he can get a water bottle and pay after Shabbos?
One may obtain foods and other supplies which are clearly for Shabbos, provided that there is no mention on Shabbos that it is a purchase, and similarly, that there is no discussion on Shabbos regarding cost, payment, weights and measures, etc. One should also not use expressions on Shabbos such as “lend me”, but rather, simply say “give it to me.” Discussions regarding payment should all take place before and/or after Shabbos. [There are more details regarding this in Shulchan Aruch OC 323:1-3, but the above is the best practice and sufficient for dealing with the above situation.]
However, there is another concern here – entering a store on Shabbos when it is open for regular trade, due to Maris Ayin. Therefore, when dealing with a store, you would need to wait outside the store and let the storekeeper bring out the water. [Of course, that would only be viable if the area outside is one in which there are no concerns of carrying on Shabbos.]
This answer assumes that the vendor is not Jewish. However, if he is Jewish but open for trade because he is not yet Frum, this should be avoided altogether, first because he might come to write down a reminder on Shabbos, and also, so as not to in any way encourage his being open on Shabbos.
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