This is probably not what you want to hear, but the answer is that you cannot conduct abstract-only searches on Google Scholar. You can see my answer to the question Google scholar "sort by date" returns articles from last year only? for more details (including the comments).
In brief, Google Scholar does not have the functionality to search only in article abstracts. This is most likely because the journals that hold the copyright to the articles that Google Scholar indexes are willing to let Google index their articles so that they can be easily found, but are not willing to let Google provide useful search functionality such as abstract-only searches, otherwise Google Scholar would become a fully-functional free replacement to their commercial services. Without the copyright permissions, Google Scholar cannot have the functionality that you seek.