Your app has setup I am not familiar with (start form imitating switchboard, use of macros, etc.), so I couldn't edit your app in short time. So I'm limited to explanations only.
1. It will be better for you to avoid multi-word names for any objects. So e.g. instead 'Seva Details table' table use tblSevaDetails, instead 'Seva Details table' form use fSevaDetails, instead 'Seva item' as name for combobox in fSevaDetails use cbbSevaItem, and instead Amount as name of textbox control linked with Amount field in fSevaDetails use txtAmount (to avoid confusion are you referring to table field or to form control).
2. For all tables, have primary key defined. The easiest way for this is to have an autonumeric field in every table as primary key. An example how this is essential: Currently your table 'Seva Items' table, or tblSevaItems as I adviced, has structure Seva Items, Cost. Let's assume after some time you have used your app, you realized you made a typo when entering a name for item, and now you have several hundred or thousand entries with faulty name in tblSevaDetails. When you correct the item name in tblSevaItems, you have to manually edit all those faulty entries in tblSevaDetails (otherwise there will be records referring to non-existing item). When you have for tSevaItems the structure like ItemID, ItemName, Cost, and in tSevaDetails ItemID is stored instead of it's name, this problem doesn't exist. (I have to continue later!)
3. You have to remove the Lookup from tblSevaDetails. In Design mode, select field Amount, and in field properties Lookup tab, select Text Box. Save changes.
4. Open form fSevaDetails in design mode. Activate combo box cbbSevaItem, (activate item properties when not activated jet,) and add AfterUpdate event for this combo. Enter the code for event - something like
Code:
Private Sub cbbSevaItem_AfterUpdate()
Me.txtAmount = Me.cbbSevaItem.Column(1)
End Sub
Me.cbbSevaItem.Column(0) returns item name (with your current setup without autonumeric ID), Me.cbbSevaItem.Column(1) returns current item price. The updated price is saved to tblSevaDetails when the record is saved (i.e. when Save command is given, the app is closed, or when another record is selected).
When you design items table as tblSevaItems: ItemID, ItemName, CurrPrice, then:
a) you have to design item details table as tblSevaDetails: DetailsID, Date, ..., Time, ItemID, Amount, ...;
b) combo's RowSource in fSevaDetails will be 'SELECT * FROM tblSevaItems;
c) combo's Source = ItemID;
d) combo's BoundColumn will be 1;
e) combo's ColumnCount = 3
f) combo's ColumnWidths = '0,2.5,0'
g) in AfterUpdate event of combo in fSevaDetails, the row to store amount will be Me.txtAmount = Me.cbbSevaItem.Column(2).
This all is on fly! I could edit your tables and forms, but I couldnt open the form in Forms view (the form was closed immediately) - so I couldn't check the code.