Buying things from the grocery store and delaying payment is not regarded as riba at all.
That is because the types of things narrated from the Prophet (peace and blessings of Allaah be upon him) which may be subject to riba are six things: gold and silver (to which cash is added nowadays), and dates, wheat, barley and salt.
The scholars divided these types into two categories, according to the reason for which riba is used:
1 – Gold and silver, etc.
2 – Wheat, barley, dates and salt.
If a person wants to buy something from the second category in return for something from the first category, such as if he wants to buy dates for cash, then there is no riba involved, and it is permissible to buy it with a delay in payment, according to scholarly consensus.
The Prophet (peace and blessings of Allaah be upon him) bought some barley from a Jew, and delayed payment of the price, and he left his shield with him as collateral.
It was narrated that ‘Aa’ishah (may Allaah be pleased with her) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) bought some food on credit from a Jew, and he gave him a shield of his as collateral (rahn). Narrated by al-Bukhaari (1999) and Muslim (1603).
Al-Bukhaari narrated it from Anas (may Allaah be pleased with him) in the words: The Prophet (peace and blessings of Allaah be upon him) left his shield as collateral with a Jew in Madeenah and took some barley from him for his family.
Al-Bukhaari included this report in a chapter entitled “The Prophet (peace and blessings of Allaah be upon him) bought on credit.”
Ibn Hajar said:
Ibn Battaal said: Buying on credit is permissible according to scholarly consensus.
Fath al-Baari (4/302).
So it is permissible for you to buy things from the grocery store and delay payment of the cash, and that does not come under the heading of riba al-nasee’ah.
And Allaah knows best.