Is the prohibition in the verse, "O you who have believed, say not 'Ra'ina' but say 'Unzurna'," general for all Muslims and in all ages, or is it specific to the address of the believers with the Messenger, may Allah's prayers and peace be upon him?

The prohibition against saying "Ra'ina" (راعنا) is general in its origin, but in the verse, it came for a specific reason, which was to avoid the Jews cursing the Prophet, peace and blessings be upon him. If this reason is absent, as is the case in Arabic dialects that use the word when haggling during shopping, then the prohibition does not apply to it.