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Install Opportunity to Receivable package from https://login.salesforce.com/packaging/installPackage.apexp?p0=04to0000000MZUv04to0000000MZV0 (v1.3)
Setup
- Goto Setup -> Opportunity Page Layout
- Select the Layout you need to provide access to this feature
- Add "Create Sale Order" and "Create Invoice" buttons to the page layout
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