The tax deductible amount is called the Receipt Amount in Altru and in the Acknowledgement Process. The receipt amount field lives on a payment record in Altru. If you are acknowledging both back office payments and sales orders, you will need to pull the receipt amount differently:
  1. In one of your acknowledgement letters, beneath "Select recipient information to use to personalize your letter," click Edit
  2. The Acknowledgement export definition will pop. To avoid duplication, in the Selected fields column, highlight the Revenue node: User-added image
  3. In the left column, expand Revenue Marketing, then expand Revenue.
  4. For back office transactions, drag Receipt Amount from the middle column, to Selected fields. With this field highlighted in Selected fields, click the pencil icon to rename this field to help you identify it as the receipt amount for back office transactions. For example: "Back office Tax Ded Amt." 
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  5. For sales transactions, you will need to pull the receipt amount from the payment on the sales order. In the left column beneath Revenue, expand Sales Order, expand Sales Order Payment, then highlight Revenue. From the middle column, drag Receipt Amount to selected fields (when you add this field, you will see two export criteria pop. You can click OK to both of these windows). Next, we recommend renaming this field as well to help identify it as the receipt amount for sales order transactions. For example: "Sales Tax Ded Amt." 
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*Note: In your acknowledgement letter, you can put these fields in one right after the other. The correct tax-deductible amount will pull in, and the other merge field will pull in as a $0.00 amount. You will need to delete the "$0.00" before sending the letter to constituents. 

Another option is to configure an additional acknowledgement letter, and adjust the revenue acknowledgement queries so that one letter acknowledges sales orders and the other letter acknowledges back office payments. Then, the corresponding merge fields can be added to each letter appropriately.